Dynamically switched voltage screen

ABSTRACT

This invention teaches an apparatus and method for determining a more efficient quality assurance or reliability test screen without falsely rejecting, i.e., over stressing, short channel length devices during voltage stress test screening. Short channel lengths devices fabricated on a semiconductor wafer have a higher tendency to fail at voltage levels that would otherwise not harm long channel length devices. The failures, however, are not related to device defects. Protection to the more vulnerable devices is provided by determining the speed of the die prior to the voltage test screen, thus, segregating the devices based on operational speed performance. Next, a lower voltage is effectively applied during wafer probe test to the faster devices, which directly correspond to the population of short channel devices. A preferred measurement for device speed entails measuring the drain-to-source current of each FET, and dividing the resultant sum by the device gate channel width. The devices with the higher values represent the faster devices. Alternatively, since faster devices draw more current, the supply current specification may be adjusted based on operational speed measurements, for fast and slow devices accordingly.

This application is a division of application Ser. No. 10/285,960, filedNov. 1, 2002, now U.S. Pat. No. 6,822,471, which was a division ofapplication Ser. No. 09/738,553, filed Dec. 15, 2000, now U.S. Pat. No.6,512,392, which was a division of application Ser. No. 09/062,043,filed on Apr. 17, 1998, which is now abandoned. The entire teachings ofthe above applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to the production techniques ofintegrated circuits and, more particularly, to a dynamically switchedvoltage screening method for quality assurance testing of integratedcircuit dies on a wafer.

2. Description of Related Art

Integrated circuit (IC) chips or dies fabricated on the same wafer havea wide range of performance in critical parameter characteristics due,in a large part, to process variations. As a result, out-going qualityand reliability is compromised by the inconsistent performance of ICchips from the same wafer lot. Typically, stress testing is performed onthe wafer during this fabrication process to eliminate the weaker chipsfrom entering the next phase of the production cycle. For example, oneof the tests regarded as one of the most severe, is that of subjectingthe devices to particularly high temperatures; typically between 85° C.and 150° C., for accelerating the infant mortality of chips in a wafer.This test, commonly referred to as “burn-in”, has the objective ofstimulating the failure of those devices which have developed somedefects during the fabrication process and/or during handling. However,this test does not segregate parts based solely on performancecharacteristics. Rather, the stress is uniformly applied to all chips ofa semiconductor wafer. Additionally, this test is time consuming,equipment intensive, and costly to perform.

Alternatively, the industry has administered a voltage stress test forimplementing a screen with less degrading effects. A voltage screen isbasically a higher than normal voltage applied during wafer test thateffectively causes defects to manifest as failures in IC chips duringsubsequent verification or operational testing. The problem associatedwith applying a higher than normal voltage level across a semiconductordie is that some dies processed with short channel lengths have a highertendency to fail when exposed to higher voltages. These short channellength devices would not otherwise be failures except for theirvulnerability to high voltage exposure. The failures are not related todefects, rather, to the over stressing of the short channel lengths inthe die. Thus, as applied, the voltage screen can be responsible forfalse failures, an undesirable quality assurance test result.Nevertheless, the industry standard has been to continuously apply avoltage stress at one voltage level to all die on a semiconductor wafer.

As discussed by Lee and Sonoda, in “TEST SYSTEM FOR NARROWING THE RANGEOF PERFORMANCE CHARACTERISTICS OF MONOLITHIC INTEGRATED CIRCUITS”, IBMTechnical Disclosure Bulletin, Vol. 15, No. 4, September 1972, somecircuits, such as those employed with FET technology, have performancecharacteristics that can differ by as much as 100% due to tolerances inthe threshold voltage. Because of these wide differences, there are atone end of the distribution curve chips exhibiting fast response timeand high power dissipation, and at the other end of the distributioncurve chips having slower circuit response time and lower powerdissipation. Thus, sorting the wafer at the IC chip level during qualityassurance or reliability testing would be advantageous to identifyingthe more vulnerable short channel length devices and exposing them toless stress, thus, eliminating false failures during voltage screening.

In the prior art, parts have been screened during wafer testing toclassify individual IC chips at various speeds. In U.S. Pat. No.5,196,787 issued to Ovens et al. on 23 Mar. 1993, entitled, “TESTCIRCUIT FOR SCREENING PARTS”, a test circuit was developed on the die tomeasure the DC characteristics of a device, which in turn, enabled oneto estimate the AC characteristics. The AC characteristic estimationswere then used to screen parts into various speed classes. However, nosuggestion is made to dynamically switch or adjust the stress testlevels based on the operational parameters measured.

Another method for determining the operational speed of an IC chip isdisclosed in U.S. Pat. No. 5,099,196 issued to Longwell et al. on 24Mar. 1992, entitled, “ON-CHIP INTEGRATED CIRCUIT SPEED SELECTION.” Byforming an oscillator in an IC semiconductor chip to generate pulsesrepresentative of the speed of other components formed in the chip, theoperational speed of the oscillator (typically, a ring oscillator), andtherefore, that of the other components formed in the semiconductorchip, can be determined. Again, the stress test levels are not alteredin response to the operation speed measurements taken.

IC chip segregation tests also include bit pattern recognition on eachdevice under test. This method is particularly useful in memory devicetesting. In U.S. Pat. No. 4,335,457 issued to Early on 15 Jun. 1982,entitled, “METHOD FOR SEMICONDUCTOR MEMORY TESTING”, semiconductormemory devices are tested using a special purpose computer which employssimple test patterns to determine the weakest bits of the device, andthen tests only these relatively few “weak bits” along with structurallyand operationally adjacent bits using highly complex test patterns todetermine if the device is functioning properly. Bit patternrecognition, however, is not a stress test screen. Thus, no adjustmentof stress test levels, predicated on the bit pattern results, is eithertaught or suggested by this prior art.

Still, other methods may be employed to distinguish the IC chips basedon variations in the operational parameters. However, independent of themethod chosen, some functionally operating IC chips continue to remainvulnerable to excessive stress test screening levels due to chip-to-chipprocess variations.

Bearing in mind the problems and deficiencies of the prior art, it istherefore an object of the present invention to provide an apparatus andmethod that determines the speed of IC chips on a semiconductor waferand adjusts the stress test levels based on the speed measured for eachdevice.

It is another object of the present invention to provide an apparatusand method for effectively protecting the short channel IC chippopulation with a lower voltage during voltage stress testing of asemiconductor wafer.

A further object of the invention is to increase the outgoing qualityand reliability of a semiconductor die using a voltage screen withoutfalsely rejecting the short channel die during voltage stress testing ofa semiconductor wafer.

It is yet another object of the present invention to provide anapparatus and method for segregating IC chips capable of a highervoltage withstand level without compromising the resultant yield fromthe wafer lot.

Another object of the present invention is to increase the measure ofreliability of the devices on a semiconductor wafer by assigning supplycurrent limits as a function of device speed.

Still other objects of the invention will in part be obvious and will inpart be apparent from the specification.

SUMMARY OF THE INVENTION

The above and other objects and advantages, which will be apparent toone of skill in the art, are achieved in the present invention which isdirected to, in a first aspect, a method for testing integrated circuitsemiconductor devices comprising the steps of: providing a wafercontaining a plurality of integrated semiconductor devices; measuring adesired parameter of the devices; and, applying a stress test to thedevices wherein test conditions of the stress test are adjusted based onthe desired parameter measurements of the devices. Measuring a desiredparameter first comprises verifying functionality of at least some ofthe integrated semiconductor devices at a set of operating conditions.The method further comprises the steps of: verifying devicefunctionality at nominal operating conditions after the stress test;and, classifying the devices as failed if the devices do not functionproperly after the stress test.

The measuring of a desired parameter comprises measuring the operationalspeed of the devices prior to applying the stress test.

Additionally, applying a stress test comprises applying a first voltageat a value higher than the device normal operating voltage to thedevices with a first measured operational speed, and a second voltage ata value lower than the first voltage to the devices with a secondmeasured operational speed, the first operational speed being less thanthe second operational speed.

The method also comprises assigning supply current specification limitsand measuring the limits of the devices such that a first supply currentspecification limit is assigned to devices with a faster of theoperational speeds and a second supply current specification limit isassigned to devices with a slower of the operational speeds, the firstsupply current limit being greater than the second supply current limit.

The current invention is directed to, in a second aspect, a method fortesting integrated circuit semiconductor devices comprising the stepsof: providing a wafer containing a plurality of integrated semiconductordevices; determining functionality of the integrated semiconductordevices at nominal operating conditions; segregating the integratedsemiconductor devices on the wafer by measuring a parameter of thedevices; applying a stress test to the devices wherein test conditionsof the stress test are adjusted based on the segregation parametermeasurements of the devices; determining functionality of the devices atnominal operating conditions after the stress test; and, classifying thedevices as failed if the devices do not function properly after thestress test.

The wafer includes devices having short and long gate channel widths,and the step of segregating the integrated semiconductor devicescomprises determining which of the devices on the wafer have short gatechannel widths. Applying a stress test pursuant to this method comprisesapplying a voltage higher than the device normal operating voltage tothe devices. The step of determining which of the devices have shortgate channel widths further comprises measuring the operational speed ofthe devices prior to applying the stress test.

Applying a stress test comprises applying a first voltage at a valuehigher than the device normal operating voltage to the devices with afirst measured operational speed, and a second voltage at a value lowerthan the first voltage to the devices with a second measured operationalspeed, the first operational speed being less than the secondoperational speed.

Measuring the operational speed of the devices may comprise using flushdelay time measurements or a performance sort ring oscillator todetermine the operational speed of the devices.

The wafer is fabricated to include n-type and p-type field effecttransistors, and wherein the step of measuring the operational speed ofthe devices comprises the steps of:

-   -   i) measuring drain-to-source current for the n-type and p-type        field effect transistors;    -   ii) determining a drain-to-source current sum by summing the        value of the n-type drain-to-source current with the absolute        value of the p-type drain-to-source current;    -   iii) determining a device ISUM value by dividing the        drain-to-source current sum by the gate channel width of the        device; and,    -   iv) segregating the devices based on the ISUM value wherein the        devices with lower ISUM values correspond to slower operational        speeds and devices with higher ISUM values correspond to faster        operational speeds.

Step (iv), segregating the devices based on the ISUM value, furthercomprises assigning devices with ISUM values less than or equal to 700μa/μm as slower operational speed devices.

The current invention provides, in a third aspect, an apparatus fortesting a wafer containing a plurality of integrated semiconductordevices including n-type and p-type field effect transistors with shortand long gate channel widths, and electrical parameters comprising:

-   -   a computer containing therein a set of instructions adapted to        perform the following functions:    -   a) measure the short and long gate channel widths of the        semiconductor devices;    -   b) segregate the semiconductor devices by the short and long        gate channel width measurements;    -   c) apply a voltage stress to the devices wherein the voltage        stress is a function of the short and long gate channel width        measurements; and,    -   d) perform a functionality test to the devices after the voltage        stress such that devices not passing the functionality test are        labeled failed devices.

In a fourth aspect, the current invention provides a program storagedevice readable by machine, tangibly embodying a program of instructionsexecutable by the machine to perform method steps for voltage screeningsemiconductor devices on a wafer, the method steps comprising:

-   -   a) determining functionality of the integrated semiconductor        devices at nominal operating conditions;    -   b) segregating the integrated semiconductor devices on the wafer        by measuring a parameter of the devices;    -   c) applying a stress test to the devices such that test        conditions of the stress test are adjusted based on the        segregation parameter measurements of the devices;    -   d) determining functionality of the devices at nominal operating        conditions after the stress test; and,    -   e) classifying the devices as failed if the devices do not        function properly after the stress test screen.

Lastly, in a fifth aspect, the current invention provides a method fortesting integrated circuit semiconductor devices comprising the stepsof:

-   -   a) providing a wafer containing a plurality of integrated        semiconductor devices including n-type and p-type field effect        transistors with short and long gate channel widths, and        operational speed parameters;    -   b) determining functionality of the integrated semiconductor        devices at nominal operating conditions;    -   c) segregating the integrated semiconductor devices on the wafer        by measuring the operational speed parameters;    -   d) applying a stress test to the devices wherein test conditions        of the stress test are adjusted based on the operational speed        measurements of the devices, such that a first voltage at a        value higher than the device normal operating voltage is applied        to the devices with a first measured operational speed, and a        second voltage at a value lower than the first voltage is        applied to the devices with a second measured operational speed,        the first operational speed being less than the second        operational speed;    -   e) determining functionality of the devices at nominal operating        conditions after the stress test; and,    -   f) classifying the devices as failed if the devices do not        function properly after the stress test.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel and the elementscharacteristic of the invention are set forth with particularity in theappended claims. The figures are for illustration purposes only and arenot drawn to scale. The invention itself, however, both as toorganization and method of operation, may best be understood byreference to the detailed description which follows taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic of a semiconductor wafer with an expanded view ofan individual die or device on the wafer.

FIG. 2A is an electrical schematic of a field effect transistor.

FIG. 2B is a structural schematic of a field effect transistor.

FIG. 3 is a graph of the distribution of the device gate length versusapplied voltage stress levels and percent of distribution.

FIG. 4 is an architectural schematic of the apparatus of the presentinvention.

FIG. 5 is graph of supply current specification as a function of slowand fast devices on a semiconductor wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing the preferred embodiment of the present invention,reference will be made herein to FIGS. 1–5 of the drawings in which likenumerals refer to like features of the invention. Features of theinvention are not necessarily shown to scale in the drawings.

In wafer probe testing of semiconductor devices, static voltage screentesting of the prior art used current limiting clamps to prevent damagefrom high current or high voltage. These methods improve reliability bytesting for device defects at a fixed “safe voltage” condition for allchips of a product type. However, using a uniform voltage during stresstest screening ultimately applies a higher than normal voltage across asemiconductor device that is processed with short channel lengths. Ingeneral, short channel lengths have a higher tendency to fail at voltagelevels that would otherwise not harm long channel length devices. Thefailures, not related to device defects, are governed by the shortchannel lengths in the die. This invention teaches an apparatus andmethod for determining a more efficient quality assurance or reliabilitytest screen without falsely rejecting, i.e., over stressing, shortchannel length devices during stress test screening. It provides thisprotection by determining the speed of the die prior to applying avoltage test screen, and effectively applying a lower voltage duringwafer probe test to the faster devices, which directly correspond to thepopulation of short channel devices. This method will enablemanufacturers to enhance the yield of IC chips from a single wafer, anddesignate more robust IC chips, those with higher voltage withstandlevels, from other IC chips fabricated on the same wafer lot.

In order to identify which dies should be packaged and which are to bediscarded based on prospective operational performance parameters whilestill at the wafer level, a test structure with corresponding dedicatedbond pads is typically incorporated in every die 2 of wafer 10illustrated in FIG. 1. The methodology of incorporating a test structureon the die for reliability and quality assurance testing has beendemonstrated in U.S. Pat. No. 5,286,656 issued to Keown et al. on 15Feb. 1994, entitled, “INDIVIDUALIZED PREPACKAGE AC PERFORMANCE TESTINGOF IC DIES ON A WAFER USING DC PARAMETRIC TEST PATTERNS”, the disclosureof which is hereby incorporated by reference. Referring to FIG. 1, aportion of wafer 10 is enlarged to show the individual integratedcircuit devices, dies, or chips 2.

Device speed is the preferred operational parameter that can be used toefficiently segregate chips more prone to voltage stress. Under voltagestress screening, the more vulnerable chips are also the more fasterchips, because the faster chips are those with short gate channellengths. There are a number of ways to determine device speed. Two wellknown methods include flush delay time measurements and performance sortring oscillator testing. Generally, any propagation delay measurementmay be employed for device operational speed distinctions.

A preferred method of determining chip speed uses a chip test structurewith n-type and p-type field effect transistors (FETs), schematicallyshown in FIG. 2A with gate 4, source 8, and drain 6. A typical fieldeffect transistor is also structurally depicted in FIG. 2B. Thepreferred operational speed measurement procedure involves measuring thedrain-to-source current, IDS, of each FET. The measured currents arethen summed; the value of the IDS current of the n-type FET added to theabsolute value of the IDS current of the p-type FET. The total sum isthen divided by the device gate channel width. This resultant isdesignated the device ISUM value. Die with lower ISUM values were shownto be the slower operating devices. These devices were also found to beable to withstand voltages above the nominal operating voltage for thegiven technology. Die with higher ISUM values represented the fasteroperational devices and sustained a significant yield loss due tonon-functionality at the higher voltage test screen level used for theslower devices.

FIG. 3 represents a distribution 20 of devices on a semiconductor waferas a function of gate length dimension and voltage. Region A representsthe proportion of operationally slower devices, i.e., the devices withlonger gate channel lengths, which typically constitute approximately85% of the devices on the wafer. These devices can withstand voltagestress screening levels on the order of 4.25 volts, as shown by line 22.Region B represents the faster devices. These devices, being morevulnerable to the applied voltage, would yield false failures at a 4.25volt stress test level. Consequently, a lower stress test voltage limitis proposed. Line 24 of FIG. 3 gives a recommended lower voltage levelof 3.75 volts for the faster devices. Testing the faster devices at thislower voltage level will decrease the probability of inducing falsefailures during the voltage stress test.

The distribution of FIG. 3 may also be used to establish a line ofdemarcation for ISUM values of the FET devices. Those devices on thegate length distribution curve with longer gate lengths, designated byregion A, generally have measured ISUM values less than or equal to 700μa/μm which correspond to slower speeds, while those devices withshorter gate lengths, designated by region B, generally have measuredISUM values greater than 700 μa/μm, which correspond to slower speeds,while those devices with shorter gate lengths, designated by region B,generally have measured ISUM values greater than 700 μa/μm,corresponding to faster speeds. The slower devices can withstand agreater voltage stress without inducing false failures. Preferably, theexact voltage level is determined by varying the voltage across adefined range such that the measured device yield drop is about 10% whenthe devices are subjected to the higher voltage levels. Thus, thedistribution curve 20 plays a significant role in selecting the stresstest limits. Importantly, this 700 μa/μm limit is governed by thetechnology of the device, and as such, represents an example of an ISUMlimit for a given technology, rather than an absolute value for alltechnology applications.

Once device speeds have been determined at the wafer final test, dynamicvoltage stress test levels are set accordingly. For example, the Cyrix686 integrated circuit can be successfully dynamically tested at 4.2volts for chips with speeds that exceed 6.5 ns, while 3.6 volts is usedfor chips with speeds less than or equal to 6.5 ns.

Referring to FIG. 4, the architecture of the apparatus used to implementthe current invention consists of an automated test equipment (ATE)system, which includes in part, a computer controller 52 and memorydevices 40. Two ATE systems that can be used to perform this testinclude the ADVENTEST system and the TERYDYNE test system.

In general, the architecture of such an automated test equipment systemtypically includes: a wafer under test 10; a pin card 12 interfacing thewafer under test through electrical conductor lines 30 and 32; aparameter measurement device 15 with input test vector bus line 17 tothe wafer 10; a formatter 14 and digital comparator 16 that connect topin card 12 through test vector bus lines 34 and 36, respectively; avector generator 18 with test vector bus lines 38 that input to theformatter 14 and digital comparator 16; a failure analysis memory device40 receiving input from the digital comparator over test vector bus line42; a timing generator 44 with input lines 43 to the vector generator18; a test controller bus interface 46 with test controller I/O buslines 50; and a computer controller 52.

As this general architecture indicates, the current invention is notdedicated to any particular ATE. Any ATE with the capability of runningthe modified software and providing the functions necessary to test andrecord designated electrical parameters of each die may successfullyperform the method of the current invention.

The preferred method of the current invention comprises 1) verifying thefunctionality of integrated semiconductor devices at nominal operatingconditions; 2) segregating the integrated semiconductor devices on thewafer by measuring the operational speed of the devices; 3) applying atwo level voltage stress test to the wafer such that the higher voltageis applied to the slower devices, and the lower voltage is applied tothe faster devices; 4) verifying the device functionality at nominaloperating conditions after the stress test; and, 5) classifying thedevices as failed if they do not function properly after the stress testscreen.

Alternatively, data from the operational speed measurements discussedabove may be used to enhance the device yield, i.e., eliminate falsefailures, by helping to establish a supply current specification that isa function of device speed. Total chip current, I_(CC), is anotherreliability screen parameter commonly tested in the industry. The basictheory is that defects draw more current, and the I_(CC) test screensthese die before they are packaged. However, short channel dies, orfaster devices, naturally draw more current and can be falsely rejected.This alternate embodiment allows the applied I_(CC) specification tochange as a result of the operational speed measurements. Referring toFIG. 5, the devices with faster speeds (shorter channels) are allowed todraw more supply current, as depicted by line 26, without beingdesignated as failed, while the devices with slower speeds are assigneda lower supply current limit, as depicted by line 28.

While the present invention has been particularly described, inconjunction with a specific preferred embodiment, it is evident thatmany alternatives, modifications and variations will be apparent tothose skilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention

1. A method for testing electronic devices comprising the steps of: a)providing a plurality of electronic devices; b) determiningfunctionality of said devices at nominal operating conditions; c)segregating said devices by measuring a parameter of said devices; d)applying a stress test to said devices wherein test conditions of saidstress test are adjusted based on said segregation parametermeasurements of said devices; e) determining functionality of saiddevices at nominal operating conditions after said stress test; and, f)classifying said devices as failed if said devices do not functionproperly after said stress test.
 2. The method of claim 1 wherein saiddevice parameter measurements are based on short and long gate channelwidths, and wherein said step (c), segregating said devices, comprisesdetermining which of said devices have short gate channel widths.
 3. Themethod of claim 2 wherein the step of determining which of said deviceshave short gate channel widths further comprises measuring theoperational speed of said devices prior to applying said stress test. 4.The method of claim 3 wherein said device includes n-type and p-typefield effect transistors, and wherein the step of measuring theoperational speed of said devices comprises the steps of: i) measuringdrain-to-source current for said n-type and p-type field effecttransistors; ii) determining a drain-to-source current sum by summingthe value of said n-type drain-to-source current with the absolute valueof said p-type drain-to-source current; iii) determining a device ISUMvalue by dividing said drain-to-source current sum by said gate channelwidth of said device; and, iv) segregating said devices based on saidISUM value wherein said devices with lower ISUM values correspond toslower operational speeds and devices with higher ISUM values correspondto faster operational speeds.